Under the context that a conventional silicon-based MOS device has been faced with numerous challenges and limitations, in order to further improve an operation speed of an integrated circuit, it is in urgent need of seeking a new material and/or a new device structure to change the current situation. As for a problem of mobility degradation, some channel materials having high mobility have been employed. A germanium material has obtained wide attentions on the grounds that its electron and hole mobility are greater than that of a silicon material and a fabrication process of the germanium-based device is compatible with a conventional CMOS process. Currently, however, not only that the fabrication technology of the germanium-based device has not matured yet and performance of the device is not adequately stable, but also for a large-scaled integration for the germanium-based MOS device in future, isolation for the device is a critical issue of acquiring excellent performance. However, up to now there has been no suitable material to be used in the isolation for the germanium-based device yet. In addition, an etching technology for the germanium material is also not quite stable.